1. Field of the Invention
The present invention relates generally to data transmission in a mobile communication system, and in particular, to a packet data transmitting device and method.
2. Description of the Related Art
For forward packet data transmission in a mobile communication system, a mobile station is assigned a forward dedicated channel (DCH) from a base station. Mobile communication systems as mentioned below include satellite systems, ISDN, digital cellular systems, W-CDMA, UMTS, and IMT-2000. Upon receipt of the forward packet data, the mobile station determines whether the reception is successful and if it is, the mobile station transmits the packet data to its higher layer. On the other hand, if errors are detected from the packet data, the mobile station requests its retransmission by the HARQ (Hybrid Automatic Repeat Request) Scheme. HARQ is a retransmission scheme using both FEC (Forward Error Correction) and ARQ (Automatic Repeat Request) for requesting retransmission of a data packet having an error. HARQ increases transmission throughput and improves system performance by channel coding for error correction. The main channel coding methods are convolutional coding and turbo coding.
A HARQ system uses soft combining to increase throughput. There are two types of soft combining: packet diversity combining and packet code combining. These are also referred to as soft packet combining. Despite having lower performance characteristics relative to packet code combining, packet diversity combining is widely used when performance loss is low, due to its simple implementation.
A packet transmission system uses packet code combining to improve transmission throughput. A transmitter transmits a code with a different data rate at each packet transmission. If an error is detected in the received packet, a receiver requests a retransmission and performs soft combining between the packet and a retransmitted packet. The retransmitted packet may have a different code from that of the previous packet. The Packet code combining is a process of combining received N packets with a code rate R to a code with an effective code rate of R/N prior to decoding, to thereby obtain a coding gain.
With regard to packet diversity combining, the transmitter transmits a code with the same code rate R at each packet transmission. If an error is detected in the received packet, the receiver requests a retransmission and performs soft combining between the packet and the retransmitted packet. The retransmitted packet has an identical code to that in the previous packet. In this sense, packet diversity combining can be considered symbol averaging on a random channel. The packet diversity combining scheme reduces noise power by averaging the soft outputs of input symbols and achieves a diversity gain as offered by a multipath channel because the same code is repeatedly transmitted on a fading channel. However, packet diversity combining does not provide such an additional coding gain as obtained according to a code structure in the packet code combining scheme.
Due to implementation simplicity, most packet communication systems have used the packet diversity combining scheme, which is under study for application to the synchronous IS-2000 system, especially for packet data transmission, and the asynchronous UMTS (Universal Mobile Telecommunication System) system. The reason is that existing packet communication systems have used convolutional codes, and even packet code combining does not offer a great gain when convolutional codes with a low data rate are used. If a system with R=1/3 supports retransmission, there is not a wide difference in performance between packet code combining and packet diversity combining. Thus, packet diversity combining is selected considering implementation simplicity. However, the use of turbo codes as FEC codes requires a different packet combining mechanism because the turbo codes are designed as error correction codes to perform very close to the “Shannon Channel Capacity Limit”, and their performance varies obviously with the coding rates unlike convolutional codes. Therefore, it can be concluded that packet code combining is feasible for a packet communication system using turbo codes in a retransmission scheme to achieve the goal of optimum performance.